Adaptive current limiter for wireless modem

Abstract

Method and apparatus for preventing current overdraw by a wireless modem sinking current from a host power supply of a host device (e.g. laptop computer or personal digital assistant (PDA)). The methods and apparatuses are applicable, but not limited to Personal Computer Memory Card International Association (PCMCIA) wireless modems and Global System for Mobile Communications/General Packet Radio Service (GSM/GPRS) wireless networks.

1. A method of limiting the current draw of a PC card wireless modem from a host power supply, said method comprising:
 measuring a current drawn by a PC card wireless modem from a host power supply; 
 converting the measured current draw to a voltage representative of the measured current draw; 
 comparing the voltage representative of the measured current draw to an adjustable reference voltage; and 
 adjusting said adjustable reference voltage as a function of one or more of host power supply capability, number of transmission slots, and RF operational band in order to establish a maximum permissible current that may be drawn by the wireless modem from the host power supply. 
 
   
   
     2. The method of  claim 1  wherein the maximum permissible current that may be drawn by the wireless modem from the host power supply is set depending on a current supplying capability of the host power supply. 
   
   
     3. The method of  claim 1  wherein the PC card wireless modem comprises a PCMCIA compatible PC card. 
   
   
     4. A current limiter for a wireless modem, comprising:
 a current sensor configured to measure a current drawn by a wireless modem from a power supply of a host device; 
 a current-to-voltage converter coupled to the current sensor and operable to convert the measured current drawn by the wireless modem to a voltage representative of the measured current draw; 
 a dynamically adjustable reference voltage generator operable to generate a reference voltage defining a maximum current that the wireless modem may draw from the host device power supply, said reference voltage being a function of one or more of host power supply capability, number of transmission slots, and RF operational bands; and 
 a voltage comparator coupled to both the current-to-voltage converter and the dynamically adjustable reference voltage generator operable to compare the voltage representative of the measured current draw to the reference voltage and generate a current overdraw signal when the voltage representative of the measured current draw is greater than the reference voltage. 
 
   
   
     5. The current limiter of  claim 4  wherein the voltage generator comprises:
 a central processing unit (CPU) having an interrupt input configured to receive the current overdraw signal and a digital output; and 
 an analog-to-digital converter (DAC) configured to receive a digital output signal from the CPU digital output and convert it to the reference voltage. 
 
   
   
     6. The current limiter of  claim 4  wherein the current overdraw signal is used to control the amount of current the wireless modem may draw from the host power supply. 
   
   
     7. The current limiter of  claim 4  wherein the wireless modem comprises a PCMCIA compatible PC card. 
   
   
     8. A wireless modem, comprising:
 a current sensor configured to measure a current drawn by the wireless modem from a power supply of a host device; 
 a current-to-voltage converter coupled to the current sensor and operable to convert the measured current drawn by the wireless modem to a voltage representative of the measured current draw; 
 a reference voltage generator operable to generate a reference voltage defining a maximum current that the wireless modem may draw from the host device power supply; and 
 a voltage comparator coupled to both the current-to-voltage converter and the reference voltage generator operable to compare the voltage representative of the measured current draw to the reference voltage and generate a current overdraw signal when the voltage representative of the measured current draw is greater than the reference voltage, 
 wherein said reference voltage generator is dynamically adjustable during operation of the wireless modem to vary the reference voltage defining the maximum current that the wireless modem may draw from the host power supply, and is a function of one or more of host power supply capability, number of transmission slots, and RF operational bands. 
 
   
   
     9. The wireless modem of  claim 8  wherein the voltage generator comprises:
 a central processing unit (CPU) having an interrupt input configured to receive the current overdraw signal and a digital output; and 
 an analog-to-digital converter (DAC) configured to receive a digital output signal from the CPU digital output and convert it to the reference voltage. 
 
   
   
     10. The wireless modem of  claim 8  wherein the current overdraw signal is used to control the amount of current the wireless modem may draw from the host power supply. 
   
   
     11. The wireless modem of  claim 8  wherein the wireless modem is PCMCIA compliant. 
   
   
     12. The method of  claim 1 , further comprising generating a current overdraw signal when the voltage representative of the measured current draw exceeds the adjustable reference voltage. 
   
   
     13. The method of  claim 12  wherein the current overdraw signal is used to control the amount of current the PC card wireless modem may draw from the host power supply.